Swinging inductors, often also referred to as swinging chokes, exhibit a relatively large inductance at light load and a progressively smaller inductance as the load increases. This makes them well suited for applications requiring good output regulation in the presence of variable load conditions. Switching power supplies and electronic ballasts are typical examples.
For such applications, swinging inductors offer a good practical compromise between designing for maximal load, in which case the inductance may be too low to meet the ‘critical’ inductance required at light load (i.e. that inductance necessary to prevent the inductor current from going to zero) and which may result in increased ripple on the output, and designing for increased inductance, which may result in a physically large inductor that is overspecified for the nominal load of the application.
To achieve variable inductance with DC bias (swinging choke), it is known practice to step-gap ferrite cores. Background prior art can be found in U.S. Pat. No. 5,816,894: Gap-providing ferrite core half and method for producing same; EP 0,518,421: Inductive device; U.S. Pat. No. 4,728,918: Storage coil with air gap in core; and U.S. Pat. No. 5,440,225: Core for coil device such as power transformers, choke coils used in switching power supply. Background information may also be found in S. T. S. Lee et al, “Use of Saturable Inductor to Improve the Dimming Characteristics of Frequency-Controlled Dimmable Electronic Ballasts”, IEEE Transactions on Power Electronics, vol. 19, no. 6, pp.1653-1660, 2004.
Further background prior art can be found in U.S. Pat. No. 5,440,225 A (Kojima) FIGS. 1-8, US 2003/0048644 A1 (Nagai et al) FIGS. 2A-2E, EP 0577334 A2 (AT&T) FIGS. 1-6D and col. 1, lines 3-9, U.S. Pat. No. 3,603,864 A (Thaker et al) FIGS. 1-5, U.S. Pat. No. 3,942,069 A (Kaneda) FIGS. 11(A)-11(E), and U.S. Pat. No. 5,847,518 A (Ishiwaki) FIG. 11.
By way of example, FIG. 1 depicts a step-gap “E” core swing choke 100 according to the prior art (Technical Bulletin: Step-gap “E” core swing chokes, Bulletin FC-S4, Magnetics, Inc (Div. Spang & Co.), 2001). At low inductor current, the thinner part of the air gap (G1) dictates a high inductance value. As the current is increased, the core at G1 progressively saturates magnetically and the inductance will ‘step’ to a lower value determined by the thicker part of the air gap (G2). The current can then be increased substantially with only a slight drop in inductance before the core at G2 becomes saturated and the inductance is effectively reduced. It is also known to use a sloped step to create a more smoothly varying inductance, as described in U.S. Pat. No. 6,657,528: Slope Gap Inductor for Line Harmonic Current Reduction.
FIG. 2 depicts an alternative core structure 200 according to the prior art in which stepped portions formed on the two central legs of “E” core segments 202, 204 are contacted to form an ungapped section 206. The working principle is similar to that described above. Thus, the ungapped section 206 of the core saturates first as DC bias current is applied to a winding (not shown) around the central section. Typically, the ungapped area might be half the total area of the central leg in order to ensure consistency in manufacture, whereby the gap is typically cut with a pair of parallel grinding wheels. However, larger ungapped sections reduce the cross-sectional area of the core available for energy storage at higher DC bias, i.e. gapped sections 208. It is therefore desirable in some circuits to minimise the ungapped cross section, as shown in FIGS. 3a and 3b, in which comparable reference numerals are used to those applying to FIG. 2, but in the range 300 to 308. As is apparent, the inductor core structure 300 is generally similar to that of FIG. 2, but has a narrower ungapped section 306 relative to ungapped section 206, with consequent widening of gapped sections 308 relative to gapped sections 208.
However, narrower ungapped sections are prone to misalignment when the cores are assembled onto a bobbin, with resulting inconsistencies in manufacture. This is shown graphically in FIG. 4. As before, comparable reference numerals are used to those applying to FIGS. 2 and 3, but in the range 400 to 408. Misalignment may occur even though care is exercised to initially assemble the core elements in precisely aligned relation.
Due to the misalignment of the two core halves 402, 404, the contact area of the stepped sections 406 is reduced, thereby lowering the inductance of the core assembly and failing to achieve the desired inductance properties. This is especially the case at light load, for which the inductance of a device with misaligned core halves might be less than half that of a corresponding device with fully aligned core halves. The problem is exacerbated with smaller cores, largely due to tolerances of core and bobbin dimensions, where a bobbin might accept a range of core halves varying in size by around ±10%. Those core halves at the smaller end of this range may not securely engage together and could therefore slip out of position. Since the core's low load inductance properties depend on the contact area (or relative closeness, as the case may be for a fully gapped structure) of the step gap, such 10% linear variations may become detrimental.
Thus, for configurations such as those depicted in FIG. 3, if the two halves are both relatively small and become misaligned, the low load inductance value could be very similar to the high load inductance value. It will be appreciated that the misalignment shown in FIG. 4 is exaggerated for the purposes of explanation, and that even slight misalignment can result in variations of the inductance of the device.
Existing core configurations and manufacturing techniques are not entirely satisfactory at mitigating the detrimental effects of misalignment and ensuring a consistent inductor characteristic, and there is therefore a need for improved techniques.